mysql - Storing SQL field names and general SQL usage with Delphi

Question

I'm starting to write my first Delphi application that connects to an SQL database (MySQL) using the ADO database components. I wondered whether there was any best way of storing the names of the fields in the database for easy reference when creating SQL queries later on.

First of all I made them a simple constant e.g. c_UserTable_Username, c_UserTable_Password, but then decided that was not a particularly good way of doing things so I am now storing them in a constant record e.g.:

type
   TUserTable = record
     TableName : String;
     Username : String;
     Password : String;
 end;

const
UserTable : TUserTable =
    (
      TableName : 'users';
      Username : 'Username';
      Password : 'Password';
    );

this allows me to create a statement like:

query.SQL.Add('SELECT ' + UserTable.Username + ' FROM ' + UserTable.TableName);

and not have to worry about hard coding the field names etc.

I've now run into the problem however where if I want to cycle through the table fields (for example if there are 20 or so fields), I can't. I have to manually type the record reference for every field.

I guess what I'd like to know is whether there is a way to iterate though all field names at once, or singularly; or am I going about this the wrong way? Perhaps I shouldn't be storing them like this at all?

Also, I've made a “Database” class which basically holds methods for many different SQL statements, for example GetAllUsers, GetAllProducts, etc. Does that sound correct? I've taken a look at a lot of Delphi/SQL tutorials, but they don't seem to go much past showing you how to run queries.

I suppose I'm just a little lost and any help is very welcome. Thanks :)

score 3 · Accepted Answer

您还可以将查询存储为 RESOURCESTRING，这将允许在事后使用资源编辑器（如有必要）对其进行编辑。

RESOURCESTRING
  rsSelectFromUsers = 'SELECT USERNAME FROM USERS ';

您的数据库类方法效果很好。我已经在我的几个项目中做到了这一点，将接口返回到包含数据集的对象......这样做的好处是当返回的接口变量超出范围时，数据集将被关闭并清除。

score 2 · Accepted Answer

好吧，您是对字段名称进行硬编码；您只需将它们硬编码在 const 中，而不是在查询本身中。我不确定这实际上会改善什么。就遍历字段而言，试试这个：

var
  Field: TField;
begin
  for Field in query.Fields do begin
     // do stuff with Field
  end;
end;

我可能会使用 TDataModule，而不是创建“数据库”类。这与您的类几乎相同，只是它允许您在设计时交互式地设计查询。您可以将所需的任何方法放在 DataModule 上。

这也使得实例化持久 TField 变得非常容易（请参阅有关该主题的帮助），您可能会发现比使用 const 存储字段名称更符合您的喜好的解决方案。

score 1 · Accepted Answer

如果您真的要使用如图所示的数据库类，请考虑记录在 D2007 及更高版本中包含函数的能力。

例如，您的示例将变为：

type
   TUserTable = record
     TableName : String;
     Username : String;
     Password : String;
     function sqlGetUserName(where:string=''):string;
   end;

const
UserTable : TUserTable =
    (
      TableName : 'users';
      Username : 'Username';
      Password : 'Password';
    );

function TUserTable.sqlGetUserName(where:string=''): string;
begin
if where='' then result := Format('SELECT %s from %s', [userName, tableName])
else             result := Format('SELECT %s from %s where %s', [userName, tableName, where]);
end;

这使得：

query.SQL.add(userTable.sqlGetUserName);

或者

query.SQL.add(userTable.sqlGetUserName(Format('%s=%s', [userTable.userName,'BOB']));

正如您所说明的那样，我真的不建议直接使用 SQL。在我看来，你永远不应该对表进行直接的 SQL 调用。这在 UI 和数据库（不应该存在）之间引入了很多耦合，并阻止您对直接表修改设置高级别的安全性。

我会将所有内容包装到存储过程中，并拥有一个数据库接口类，将所有数据库代码封装到一个数据模块中。您仍然可以使用直接链接到数据模块中的数据感知组件，您只需在链接前加上 DM 名称即可。

例如，如果您构建了一个类似的类：

type
   TDBInterface = class
      private
         function q(s:string):string; //just returns a SQL quoted string
      public
         procedure addUser(userName:string; password:string);
         procedure getUser(userName:string);
         procedure delUser(userName:string);

         function testUser:boolean;

         procedure testAllDataSets;
      end;

function TDBInterface.q(s:string):string;
begin
result:=''''+s+'''';
end;

procedure TDBInterface.addUser(userName:string; password:string);
begin
cmd.CommandText:=Format( 'if (select count(userName) from users where userName=%s)=0 '+
                         'insert into users (userName, password) values (%s,%s) '+
                         'else '+
                         'update users set userName=%s, password=%s where userName=%s',
                         [q(userName), q(userName), q(password), q(userName), q(password), q(userName)]);
cmd.Execute;
end;

procedure TDBInterface.getUser(userName:string);
begin
qry.SQL.Add(Format('select * from users where userName=%s', [q(userName)]));
qry.Active:=true;
end;

procedure TDBInterface.delUser(userName:string);
begin
cmd.CommandText:=Format('delete from users where userName=%s',[userName]);
cmd.Execute;
end;

procedure TDBInterface.testAllDataSets;
begin
assert(testUser);
end;

function TDBInterface.testUser: boolean;
begin
result:=false;

   addUser('99TEST99','just a test');
   getUser('99TEST99');
   if qry.IsEmpty then exit;
   if qry.FieldByName('userName').value<>'99TEST99' then
      exit;
   delUser('99TEST99');
   if qry.IsEmpty then
      result:=true;
end;

您现在可以在数据接口上进行某种形式的单元测试，您已经从 UI 中删除了 SQL，并且事情正在好转。你的接口代码中仍然有很多丑陋的 SQL，所以把它移到存储过程中，你会得到：

type
   TDBInterface = class
      public
         procedure addUser(userName:string; password:string);
         procedure getUser(userName:string);
         procedure delUser(userName:string);

         function testUser:boolean;

         procedure testAllDataSets;
      end;

procedure TDBInterface.addUser(userName:string; password:string);
begin
cmd.CommandText:='usp_addUser;1';
cmd.Parameters.Refresh;
cmd.Parameters.ParamByName('@userName').Value:=userName;
cmd.Parameters.ParamByName('@password').Value:=password;
cmd.Execute;
cmd.Execute;
end;

procedure TDBInterface.getUser(userName:string);
begin
sproc.Parameters.ParamByName('@userName').Value:=userName;
sproc.Active:=true;
end;

procedure TDBInterface.delUser(userName:string);
begin
cmd.CommandText:='usp_delUser;1';
cmd.Parameters.Refresh;
cmd.Parameters.ParamByName('@userName').Value:=userName;
cmd.Execute;
end;

您现在可以将其中一些函数移动到 ADO 线程中，并且 UI 不会知道添加或删除用户是在单独的进程中发生的。请注意，这些都是非常简单的操作，因此如果您想做一些方便的事情，例如在处理完成时通知父级（例如在添加/删除/更新发生后刷新用户列表），您需要将其编码到线程中模型。

顺便说一句，添加代码的存储过程如下所示：

create procedure [dbo].[usp_addUser](@userName varchar(20), @password varchar(20)) as
if (select count(userName) from users where userName=@userName)=0
   insert into users (userName, password) values (@userName,@password)
else 
   update users set userName=@userName, password=@password where userName=@userName

另外，一点免责声明：这篇文章很长，虽然我试图检查大部分内容，但我可能在某个地方遗漏了一些东西。

score 1 · Accepted Answer

1

可能有点离题，但您可以使用RemObjects 中的Data Abstract。

于 2009-06-05T19:14:55.040 回答

score 0 · Accepted Answer

在Analyzing DataSets（在 Delphi 和 Kylix 中）获取战利品

该代码是操作表元数据的一个很好的例子。您可以获取字段名称，然后编写可以创建基本单元/或其接口部分的代码生成器。

mysql - Storing SQL field names and general SQL usage with Delphi

5 回答 5

Related

Reference